Add squash commit message in MR form

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/223029

It will add a message when the
squash commit checkbox is disabled.
It will indicate whether it is required or not.

app/views/shared/issuable/form/_merge_params.html.haml

@@ -26,3 +26,6 @@
         = label_tag 'merge_request[squash]', class: 'form-check-label' do
           Squash commits when merge request is accepted.
           = link_to icon('question-circle'), help_page_path('user/project/merge_requests/squash_and_merge'), target: '_blank'
+        - if project.squash_always?
+          .gl-text-gray-600
+            = _('Required in this project.')

spec/features/merge_request/user_edits_merge_request_spec.rb

@@ -16,6 +16,40 @@ RSpec.describe 'User edits a merge request', :js do
     visit(edit_project_merge_request_path(project, merge_request))
   end
 
+  describe 'Squash commits' do
+    it 'displays "Required in this project" for "Required" project setting squash option' do
+      project.project_setting.update!(squash_option: 'always')
+      visit(edit_project_merge_request_path(project, merge_request))
+
+      expect(page).to have_content('Squash commits when merge request is accepted.')
+      expect(page).to have_content("Required in this project")
+    end
+
+    it 'does not display message for "Allow" project setting squash option' do
+      project.project_setting.update!(squash_option: 'default_off')
+      visit(edit_project_merge_request_path(project, merge_request))
+
+      expect(page).to have_content('Squash commits when merge request is accepted.')
+      expect(page).not_to have_content("Required in this project")
+    end
+
+    it 'does not display message for "Encourage" project setting squash option' do
+      project.project_setting.update!(squash_option: 'default_on')
+      visit(edit_project_merge_request_path(project, merge_request))
+
+      expect(page).to have_content('Squash commits when merge request is accepted.')
+      expect(page).not_to have_content("Required in this project")
+    end
+
+    it 'is hidden for "Do not allow" project setting squash option' do
+      project.project_setting.update!(squash_option: 'never')
+      visit(edit_project_merge_request_path(project, merge_request))
+
+      expect(page).not_to have_content('Squash commits when merge request is accepted.')
+      expect(page).not_to have_css('#merge_request_squash')
+    end
+  end
+
   it 'changes the target branch' do
     expect(page).to have_content('From master into feature')